The Glen Canyon Dam and the Diablo Canyon Nuclear Power Plant, two of the most controversial energy projects in American history, are back in the news. Though they both generate much desired low-carbon energy, Diablo Canyon is closing and Glen Canyon may soon face the same fate.

Concerns about the climate, customer demands for new technologies, and an evolving regulatory atmosphere are transforming the US energy market. As utilities and lawmakers adapt to this energy transition, they have begun to favor more agile and more distributed energy resources. This evolution not only threatens the future of dirty coal-fired power plants, but it also has industry folks questioning the merits of other traditional forms of large-scale power generation. Nuclear and hydroelectric power sources, like those at Glen Canyon and Diablo Canyon, are climate friendly sources of energy, but they do carry other environmental risks. And, because they cannot be rapidly scaled up or down to match daily peaks in energy demand, regulators and utilities are moving on. Decades after starring in environmental firestorms, Glen Canyon and Diablo Canyon now face an unceremonious conclusion as decision-makers consider decommissioning the once-controversial facilities.

Both Glen Canyon and Diablo Canyon were major touchstones in the creation and growth of the modern environmental movement. The Glen Canyon Dam began as an afterthought, a dam across a little known gorge proposed as part of the Colorado River Storage Project in the 1950s. But when conservationist David Brower, director of the Sierra Club, learned that the project would dam the Colorado River at Echo Park and therefore flood Dinosaur National Monument, he launched a national campaign to save the park. Brower’s campaign was successful. He saved Dinosaur National Monument, legitimized a nascent environmental movement, and built momentum for the later passage of the Wilderness Act. However, to secure the protection of Echo Park, the Sierra Club consented to the construction of a larger dam at Glen Canyon and to the creation of the enormous Lake Powell, the 160,000 acre reservoir that now lies behind the dam. Today, the dam houses eight generators each with 165,000 kW capacity that create power as water from the lake tumbles 710 feet to the river below. The fight to save Dinosaur National Monument was among environmentalists first political victories but it came at the expense of the gorges and vistas at Glen Canyon, a natural beauty that Brower and other leading environmentalists would only discover after preparations for the dam had already begun.

The construction of a nuclear power plant at Diablo Canyon was the result of a similar environmental compromise, though it took a more circuitous route to completion. In the 1960s, the Sierra Club blocked construction of nuclear facilities at multiple sites in California, arguing that the locations were either earthquake risks or ecologically sensitive. To overcome this opposition, Pacific Gas and Electric (PG&E) cut a deal with the Sierra Club, promising to build the nuclear plant at the less fragile Diablo Canyon if the club agreed to end its opposition. Other environmentalists, however, refused to accept the compromise. Diablo Canyon became the target of a broad anti-nuclear movement that used direct action to block the construction and operation of nuclear facilities. Protestors staged intermittent demonstrations and blockades at Diablo Canyon from 1977 to 1982. During a two-week protest in 1981, police arrested almost 2,000 activists. The Diablo Canyon demonstrations fell short – the power plant went online in 1985 – but they were an essential part of a dramatic shift undertaken by the environmental movement. As the movement embraced direct action tactics and human health concerns in the 1970s and 1980s, huge anti-nuclear protests that stalled or prevented generation of nuclear power at Diablo Canyon in California, Seabrook in New Hampshire, and Shoreham in New York all helped cement this shift toward a new generation of environmentalism. Like the Glen Canyon activism, the protests at Diablo Canyon occurred in the midst of a transition in American environmentalism and occupy a large place in the legacy of the movement.

Today, environmentalists’ past desires are becoming a reality. Pacific Gas and Electric (PG&E) recently announced plans to shutter the Diablo Canyon Nuclear Power Plant. Citing the growth of energy efficiency goals, renewable energy requirements, and distributed energy resources, PG&E chose to the close the plant after its license expires in 2025. Working with anti-nuclear activists and labor groups, the utility drafted a plan to replace Diablo Canyon’s electricity with carbon-free energy sources like renewables, efficiency, and storage.

The future for Glen Canyon is less certain. While the dam continues to produce electricity, Lake Powell now sits only half full, cannot generate power at full capacity, and is rapidly losing water to evaporation and leaks. These are only some of the concerns raised by Abrahm Lustgarten in a recent article the explores a growing groups of experts calling for the US Bureau of Reclamation to decommission the dam. The individuals calling to open the dam remain a vocal minority, but amidst the termination of other massive energy projects, that vocal minority is gaining a larger audience.

The potential decommissionings at Glen Canyon and Diablo Canyon are not momentous alone, but they do serve as useful reminders of the trajectory of energy and environmental discussions in the United States. First, they are a reminder of where we were. Earlier environmental concerns, both the public lands policies of the 1950s and the vehement anti-nuclear activism of the late 1970s, have been surpassed in our current era of climate consciousness. As Glen Canyon and Diablo Canyon fade from view, it is useful to reflect on how these major energy projects catalyzed environmental activism and encapsulated the overriding environmental concerns of their period

Second, Glen Canyon and Diablo Canyon remind us where we are – on the precipice of a potential energy transition. Such dramatic shifts, like the move to coal in the 1800s or the move to oil in the 1900s, are rare and slow. Only a few years ago, another transition seemed either unlikely or distant. Thanks to the rapid acceleration of clean energy technologies and climate-related demands, the clean energy transition may already be upon us. Energy transitions are moments of opportunity but also moments of uncertainty, which is why there is no consensus on whether closing carbon-minimal sites like Diablo Canyon represent a progressive move or a window for the return of fossil fuel-fired energy.

Lastly, the conversations over Glen Canyon and Diablo Canyon remind us where we need to be going. Whether we are in the midst of an energy transition or not, today’s energy challenges demand new, creative, and environmentally conscious solutions. Will shuttering two massive energy sources help address concerns over carbon emissions, distributed resources, and peak load management? That remains to be seen.